Blight control strategies will have to change this season if potato growers are to combat the spread of a new aggressive, fungicide-insensitive and resistant strain, warns agronomists.

The dark green 37_A2 form of Phytopthora infestans has quickly spread across Europe, reaching England two years ago when five cases were reported.

Around 20 cases were officially recorded in 2017, mainly in the West Midlands, Yorkshire and Kent, and more recently in Suffolk, but agronomy firm Hutchinsons believes the actual figure could be higher and all crops are at risk no matter where grown.

The new strain is at least, if not more, aggressive than the dominant blue 13 and pink 6, but the crucial difference is that it appears equally aggressive on foliar and tuber blight, warns the company's root crop technical manager Darryl Shailes.

He said: “There was a massive failure of fluazinam at the Eurofins blight trials last year and dark green 37 was implicated in several cases of store breakdown.”

Furthermore, dark green 37 has reduced sensitivity to fluazinam, which is in many fungicides often used towards the end of the season to reduce tuber blight.

He continued: “Fluazinam has been a cost-effective option for tuber and foliar blight, especially against the blue 13 strain, but until we have access to real-time information to determine the exact genotype of blight spores present in the field, we have to question its position in blight programs.

“The yield loss from foliar infection of blight is often very small except in the worst years but the consequential losses from tuber blight can be massive.”

Dark green 37 is most likely to have reached the UK in imported seed, but also spreads via airborne spores, says John Keer of Richard Austin Agriculture.

“We expect a progression of the new strain from hotspot areas this year, but it’s hard to predict where we’ll see it. Blue 13 and pink 6 took two or three years to spread and become more dominant, whereas other genotypes such as A33 have remained very localised.”

Dr Keer also says that despite a wide diversity of blight genotypes in the UK, usually only one is responsible for outbreaks in the field. All growers should therefore be wary of the new strain and not assume crops are “safe”, as fluazinam will be ineffective if dark green 37 is present, he says.

Using it risks a failure of blight control and favours selection of more resistant strains, so the focus must be on alternative chemistry.

The increasing aggressiveness of new blight strains makes it vital to manage risk from the outset, says Mr Shailes, who warns that with the right weather, blight can be present as soon as crops emerge.

All inoculum sources should be controlled, including volunteers and outgrade piles. Growing more resistant varieties affords some flexibility around blight spray timings, but with ever-changing disease populations it is only part of risk mitigation and chemical options remain essential.

“There is a myriad of materials growers can use instead of fluazinam. As with any fungicide strategy, alternate chemistry so you’re not relying on any one active ingredient or mode of action.”